The invention concerns a temperature-compensated diode rectifier for an HF level controller, which is located in the gain control loop of an HF amplifier to control the output level of an HF transmitter. The controlled HF amplifier is preferably arranged at the output of a radio telephone modulator. In particular, this is a cellular telephone designed to operate in different transmission bands. The object of the control circuit is to keep the transmitting power of the radio telephone""s antenna constant at a set value within a large power range and over a large operating temperature range.
During the telephone operation, radio telephones like cellular telephones adjust their HF transmitting power as a function of the base station density in a radio network and the momentary transmission conditions. After the radio telephone accesses the radio network, the latter assigns a transmission channel to the telephone and continuously monitors the transmission quality. In addition to other HF parameters such as channel frequency and time slot position, the level of the HF transmitting power must be maintained relatively precisely. Rural areas with a low number of simultaneous telephone connections generally have a low base station density despite the advanced development of the radio networks, so that a radio path of up to 40 km to the next base station and poor propagation conditions require the maximum available transmitting power of about 33 dBm (=2 Watt). In contrast thereto, dense population and a high base station density leads to short radio paths which are often only 100 meters long. The transmitting power in such short radio paths can often be reduced to a few dBm/mW to reduce battery load and ensure the transmission quality.
In addition, with high transmitting power and low cell size common channel disturbances can occur in other cells if in accordance with the applicable conventions both cells occupy the same channel. On the other hand, transmitting power which is too low leads to insufficient transmission quality.
It can therefore be seen that on the one hand the control circuit must adjust the HF transmitting power of a cellular telephone as precisely as possible and in a reproducible manner in accordance with the transmission conditions in a large dynamic range, and on the other hand it must maintain the set value in a wide range of operating temperatures. These requirements are difficult to fulfill with low HF transmitting power.
According to the present state of development, the transmitting power including power losses in the cellular telephone, for example in the transmitting/receiving switch, must be adjustable for a 900 MHz frequency band in a range of levels between 5 dBm and 39 dBm, and for the frequency bands of 1800 MHz and 1900 MHz in a range between 0 dBm and 36 dBm. These settings should have an accuracy of xc2x10.5 dBm in the temperature range of xe2x88x9220xc2x0 C. to +60xc2x0 C.
The present invention starts with a temperature-compensated diode rectifier circuit according to application EP 0 834 987 A2. To better understand the problem, differences between the known solution and the present invention are explained by means of FIG. 1.
An HF amplifier PA is located between the modulator output of a not illustrated telephone module and an antenna circuit A, to amplify a transmission signal RFIN which in this case is digitally modulated to 900 MHz on a carrier. To obtain an HF output signal RFOUT for the antenna circuit A at a defined power level in the above cited level range, a control loop with a comparator COM which compares the known desired value to the actual value, adjusts the amplification of the HF amplifier PA. The comparator COM compares a d.c. control voltage UCTR, which corresponds to the desired power level, with a rectified voltage UD whose level depends on the power level of the HF output signal RFOUT. A rectifier circuit with a rectifier diode D1, a charging capacitor C1 and a ballast resistor R2 obtains the rectified voltage UD from the output signal RFOUT of the antenna circuit. A directional coupler D-CO connects the rectifier circuit to the antenna circuit.
The voltage of the HF output signal RFOUT in the cited level range varies by more than the ratio n=1:30. This requires a linear conducting characteristic of the rectifier diode and a steep transition from the off mode to the conducting mode, in order to obtain a small setting deviation even at low levels. This is made more difficult because in extreme cases only a minimum 3 V operating voltage is available for a cellular telephone. Since the comparator COM can only compare voltages which are below its own operating voltage, the directional coupler D-CO and the rectifier circuit must be designed so that even at a maximum power level the rectified voltage UD is still at least slightly under this value. The result is that even with an ideal characteristic diode curve, the part of the rectified output signal ÛRF at the lowest power level in the rectified voltage UD would be under 100 mV.
The transition from the off mode to the conducting mode in rectifier diodes lies outside of the axial zero point. Without special measures only one rectified voltage UD reaches the output when the amplitude of the decoupled output signal is above a threshold voltage UT. For that reason the diode D1 operates advantageously with a bias voltage, or like the solution of application EP 0 834 987 A2 with a bias current. The threshold voltage UT depends on the operating temperature and in Schottky diodes for example it is in the desired temperature range under UT=300 mV.
The power level at the output of the HF amplifier PA must be independent of operating temperature changes. However this is not so in conventional rectifier circuits. Conventional diodes with small forward currents have a temperature dependence of about 2.0 mV/xc2x0 C. Thus a permissible 80xc2x0 C. temperature change requires a change of 160 mV in the rectified voltage UD. This is larger than the part of rectified the output signal ÛRF in the voltage.
To compensate the temperature run of the rectified voltage UR, it is known from application EP 0 834 987 A2 to place a compensating diode D2 in series with the ballast resistor R2, and to connect one side of the rectifier diode D1 to the battery voltage UBAT via a ballast resistor R1. This creates a voltage divider for the battery voltage UBAT with a series circuit of a diode D1 or D2 and a resistor R1 or R2 in each dividing branch, and the rectifier output OR has the rectified voltage UD=UDO+ÛRF which contains a bias component UDO=UT+ÛR2. With a bias current IBIAS between 20 xcexcA and 200 xcexcA, the battery voltage ÛBAT places both diodes D1, D2 in the conductive condition. Due to the temperature dependence of diodes D1 and D2, the bias current IBIAS is also temperature dependent. However since equal diode types and thermally coupled diodes D1, D2 have the same temperature dependence in each dividing branch, the division ratio of the voltage divider remains stable insofar as the dropping resistor R1 and the ballast resistor R2 have the same value and the comparator COM has a high input resistance. In this case the bias component UDO=1/2 UBAT and only depends on the battery voltage UBAT level. The rectified voltage UM can therefore only vary between full and half the operating voltage value. In practice this reduces the variation range of the rectified output signal ÛRF in the rectified voltage UD to 1.5 Volts. However if the resistors R1 and R2 are different, for example to expand the variation range of the rectified voltage UD, the rectifier circuit has a defined temperature run. The ratio of the resistors to each other can be used to establish a positive or a negative temperature run. The decoupled output signal goes from the directional coupler D-CO via a coupling capacitor C2 to a summation point S in the rectifier circuit where it is superimposed on the bias current IBIAS. The resistor RE is used to adapt the directional coupler D-CO and terminate the wave impedance.
The bias component UDO is always at a fixed ratio with the battery voltage UBAT, which can fluctuate broadly in a radio telephone. For that reason the level of the d.c. control voltage UCTR must also be coupled to the changing battery voltage UBAT.
In addition to the cited disadvantages, it was also shown in practice that at small power levels up to about 10 dBm the known rectifier circuit only produces a rectified output signal ÛRF of a few millivolts at the rectifier output, so that the control circuit operation at the lower level of the HF output signal RFOUT is insufficient. The cause of this deficiency is apparently the linkage of the bias current IBIAS with the decoupled part of the output signal RFOUT at the summation point S. By comparison with the resistors R1 and R2, the diodes D1 and D2 only have a small impedance. This means that the resistors R1 and R2 act as a current source with respect to the diodes and provide a relatively stable bias current IBIAS above the transition from the off mode to the conducting mode, so that the amplitude of the decoupled HF output signal RFOUT has to exceed a minimum power value to compensate this current and with the diode D1 achieve a rectified voltage that depends on the HF output signal RFOUT. Until the bias current IBIAS is compensated, the diodes D1, D2 have a small impedance and the capacitors C1 and C2 form an a.c. voltage divider which divides the decoupled HF output signal RFOUT accordingly. This means that instead of the rectified output signal ÛRF, at a low level only a component of the a.c. voltage takes place in the rectified voltage UD. Thus the bias current in the known circuit first realizes the above described temperature compensation, but provides insufficient bias to the diodes D1, D2 with the threshold voltage UT.
Starting with the cited deficiencies of the known solution, it is the object of the invention to create a rectifier circuit in a control circuit for regulating the level of an HF output signal in an antenna circuit, which by comparison with the known solutions also has sufficient sensitivity at a low HF output level in addition to good temperature compensation. Beyond that the rectifier circuit must also work with devices designed to operate in different frequency bands, in so-called dual band mobile phones.
To achieve this object the invention starts with a rectifier circuit containing a rectifier input for the HF output signal which is decoupled from the antenna circuit, and a rectifier output for a rectified voltage. To measure the level of the HF output signal, the circuit is coupled to the rectifier input via a directional coupler in the antenna circuit. The rectifier circuit contains a rectifier diode, a charging capacitor and a ballast resistor which leads from the rectifier output to a ground connection of the circuit. The rectifier input is connected to a d.c. input voltage to stabilize the rectified voltage against temperature influences. Furthermore a compensating diode is placed in series with the ballast resistor, and a dropping resistor in series with the rectifier diode. A voltage divider, in which each dividing branch contains a resistor and a diode that is polarized in the flow direction, leads from the d.c. input voltage to the rectifier output, so that the rectified voltage is independent of the temperature and preferably contains a bias component at half the level of the d.c. input voltage.
In contrast to the known solution however, the components of the rectifier circuit and the directional coupler of the invention are connected to the d.c. input voltage so that the voltage amplitude of the decoupled HF output signal is added to the d.c. input voltage, and the dropping resistor of the rectifier diode is located between the charging capacitor and the rectifier output.
Another measure for achieving the object of the invention is that the d.c. input voltage is stabilized and is only slightly higher than twice the threshold voltage of a diode.
The addition of the HF output signal is achieved in that on one side the connections of the directional coupler are directly connected to the d.c. input voltage, and on the other side directly to the rectifier input, so that the inductively decoupled HF a.c. voltage is in series with the d.c. input voltage.
This has the advantage that on the one hand the decoupled HF a.c. voltage at the rectifier input is directly superimposed on the d.c. input voltage, and works against it when the amplitude of the HF output signal is negative, while no signal current flows. On the other hand the rectified part of the output signal in the rectified voltage at the charging capacitor is nearly twice as large as the rectifier output. With a small HF output level, the cited measures allow better use of the current-voltage dependence of the rectifier diode during the transition from the off mode to the conducting mode. The small d.c. input voltage enables to almost double the variation range of the rectified voltage and thereby also a corresponding increase in the HF a.c. voltage at the rectifier input, without the maximum rectified voltage exceeding an unallowable high value at the rectifier output. The position of the dropping resistor also enables a further doubling of the HF a.c. voltage at the rectifier input, without the rectified voltage exceeding the unallowable high value or changing the temperature run. Furthermore, since no fixed bias current is fed to the rectifier diode, no output power is needed to compensate when signal amplitudes are negative.
As a result and compared to the known solution, the diode rectifier circuit of the invention provides sufficient sensitivity with equally good temperature compensation in an HF output level range which is expanded downward by about 10 dBm. This diode rectifier circuit also allows a control circuit of the HF transmitter to control output levels in the 0 dBm to 10 dBm range with sufficient accuracy.